The present invention relates generally to inertial sensors, such as heated air mass accelerometers, gyros, and the like, and more particularly to an air mass accelerometer sensor cell structure including a differential thermocouple and a heater which is adjacent to a poly (polycrystalline) silicon element that is common to a pair of thermocouples of the differential thermocouple.
In order to measure acceleration, it is necessary to have a mass which moves relative to a motion sensor. The most common low-cost method utilizes conductive, closely spaced fingers, the mutual capacitance of which changes when the motion sensor is subjected to acceleration. The major problem of such closely spaced conductive fingers is stiction that occurs under extreme acceleration. To avoid this problem, some prior inertial sensors utilize a heated air parcel as the mass, and measure the magnitude of the moving hot air mass on the temperatures of different thermocouples/thermopiles, and measure the resulting differential temperature change in the thermocouple/thermopile junctions to determine the acceleration.
FIG. 1 shows a section view diagram of a single conventional heated air mass accelerometer or inertial sensor cell 1-1 which includes a silicon substrate 2 having a dielectric region or layer 3 thereon. A cavity 9 has been etched into silicon substrate 2. The upper part of cavity 9 is bounded by the bottom surface of dielectric region 3. Dielectric region 3 is sometimes referred to as a “dielectric stack”, and includes multiple dielectric layers on which various metal and polycrystalline silicon traces, respectively, are formed. In FIG. 1, a first thermocouple 7A of a thermopile Q1 formed in region 3 includes a metal trace 4A, a tungsten contact 5A, and a poly (i.e., polycrystalline silicon) trace 6A. (A thermopile is a “stack” or “pile” of thermocouples that are electrically coupled in series.) Similarly, a thermocouple 7B of thermopile Q3 is formed in region 3 and includes a metal trace 4B, a tungsten contact 5B, and a poly (i.e., polycrystalline silicon) trace 6B. Thermocouple 7A is one of a large number of identical thermocouples in a thermopile Q1 of a conventional accelerometer sensor or inertial sensor 13-1 shown in subsequently described FIG. 2. A voltage VQ1, which is a function of the temperature of thermopile Q1, is developed across the terminals of thermopile Q1. Similarly, thermocouple 7B is one of a large number of identical thermocouples in a thermopile Q3 of conventional accelerometer sensor 13-1 shown in subsequently described FIG. 2, and a voltage VQ3 is developed across the terminals of thermopile Q3.
Accelerometer cell 1-1 in FIG. 1 also includes a sichrome (SiCr) heater element 8. An air mass 10 located in cavity 9 is heated by heater 8. If accelerometer cell 1-1 moves laterally, the inertia of heated air mass 10 causes it to tend to remain stationary. Therefore, the distance between heated air mass 10 and the thermocouple junction of one of the thermocouples 7A and 7B decreases, and the distance between heated air mass 10 and the other thermocouple junction increases. This results in a difference in the temperatures of the two thermocouple junctions, and therefore results in a difference between the thermopile output voltages VQ1 and VQ3. An optional dome 13 which contains heated air mass 11 also may be provided to improve the performance of accelerometer cell 1-1.
Thermopiles Q1 and Q3 in accelerometer cell 1-1 each typically are composed of roughly 15 series-connected accelerometer cells 1-1, generally as shown in FIG. 1.
Referring to FIG. 2, accelerometer sensor 13-1 includes circular etchant openings 30 that extend through dielectric region 3 to cavity 9 (FIG. 1) to allow it to be etched as a single large cavity that extends beneath all of the thermopiles formed in the dielectric stack 3 on semiconductor substrate 2 (FIG. 1). In FIG. 2, sichrome heaters 8 are formed around the center region of the integrated circuit chip in which accelerometer sensor 13-1 is fabricated, so as to provide heating to a single large air mass that is much larger than air mass 10 in cavity 9 shown in FIG. 1. In FIG. 2, the cavity (e.g., cavity 9 of FIG. 1) extends beneath all of thermopiles 70A,B formed in the integrated circuit chip in which accelerometer sensor 13-1 is formed.
The thermopile output voltages VQ1, VQ2, VQ3, and VQ4 in FIG. 2 are developed across all of the thermocouple pairs 7A,7B (FIG. 1) that are contained in each of thermopiles Q1, Q2, Q3, and Q4, respectively. Quadrant thermopiles Q1 and Q3 are oriented in an “x” direction, and quadrant thermopiles Q2 and Q4 are oriented in an orthogonal “y” direction. Accordingly, the acceleration in the x direction is proportional to VQ1 minus VQ3, and acceleration in the y direction is proportional to VQ2 minus VQ4. A limitation of the technique shown in Prior Art FIGS. 1 and 2 is the number of thermocouples in the thermopile and the high resistance of the thermopile. Another limitation is the high cost of the required package, which needs to have a cavity to protect the thin membrane portion of the dielectric stack 3 over cavity 9 in the accelerometer.
There potentially is a much larger market than presently exists for inertial sensors, accelerometers, etc., for example in buildings, bridges, engines, and many other applications, if inertial sensors can be provided with sufficient accuracy and at sufficiently low cost.
Thus, there is an unmet need for a much lower cost inertial sensor than is presently available.
There also is an unmet need for an inertial sensor and method which can be provided using conventional integrated circuit processing techniques without use of additional costly MEMS (microelectromechanical systems) fabrication techniques.
There also is an unmet need for an inertial sensor which can be provided using conventional integrated circuit processing techniques and which requires substantially less integrated circuit chip area than the closest prior art.
There also is an unmet need for a low-cost inertial sensor and method which provides substantially better SNR (signal to noise ratio) operation than the closest prior art.